To decrypt a database
Mark database files for decryption (using the DecryptFile or DecryptFiles action)
Decrypt the marked database files (using the ApplyPendingChanges action)
Disable database encryption (using the DisableDatabaseEncryption action)
If you run a secondary database on the same machine as the primary and you want to avoid recloning the secondary, specify RetainMasterKey=true so that there is no chance that the secondary restarts and requires the deleted master key before it replays the DisableDatabaseEncryption action.
Take a full offline backup of your database when the decryption process is complete, to avoid reprocessing the decryption during recovery.